GREAT MILLS, Md. (AP) — A teenager wounded a girl and a boy inside his Maryland high school Tuesday before an armed school resource officer was able to intervene, and each of them fired one more round as the shooter was fatally wounded, a sheriff said.

St. Mary’s County Sheriff Tim Cameron said the student with the handgun was declared dead at a hospital, and the other two students were in critical condition. He said the officer was not harmed.

Cameron said authorities were still investigating whether the shooter was killed by the officer.

“When the shooting took place, our school resource officer, who was stationed inside the school, was alerted to the event and the shots being fired. He pursued the shooter and engaged the shooter, during which that engagement he fired a round at the shooter,” Cameron said.

“Simultaneously the shooter fired a round as well. So, in the hours to come, in the days to come, through a detailed investigation, we will be able to determine if our SRO’s round struck the shooter.”

This latest shooting comes as lawmakers nationwide are under pressure to take action against gun violence following the Valentine’s Day killings of 17 people at a Florida high school by a teenager with an assault weapon. Maryland’s Senate joined the House Monday night to ban bump stocks, which enable a semi-automatic rifle to mimic a fully automatic weapon.

Outside the school, a line of about 20 school buses formed on the street in front of the building. Ambulances, fire trucks and other emergency vehicles crowded the parking lot and the street. No students or parents could be seen outside at midmorning.

The St. Mary’s County Public Schools said the situation was “contained” after the shooting at Great Mills High School, which has about 1,600 students and is near the Patuxent River Naval Air Station, about 65 miles (104 kilometers) southeast of Washington.

Agents with the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives joined deputies at the scene. The county sheriff said parents or guardians should stay away, urging them to go instead to Leonardtown High School to reunite with Great Mills students there.

Many students across the country are calling for effective gun controls, leading up to Saturday’s March For Our Lives rally in the nation’s capital against gun violence in schools. Threats against schools have proliferated as well, and Great Mills High has not been immune.

Just last month, the school’s principal, Jake Heibel, told parents in a letter posted on the local news site The Bay Net that two students were interviewed after they were overheard mentioning a school shooting, and they were found to pose no threat. Heibel said the school increased its security nevertheless after social media posts about a possible school shooting “circulated quite extensively.”

Also last month, St. Mary’s County Sheriff’s office said it arrested two teenage boys for “Threats of Mass Violence” and a 39-year-old man on related charges after the teens made threats about a potential school shooting at Leonardtown High School, a high school about 10 miles from Great Mills. Police said they obtained a search warrant that led to them finding semi-automatic rifles, handguns and other weapons, along with ammunition.

Deputies and federal agents converge on Great Mills High School, the scene of a shooting, Tuesday morning, March 20, 2018 in Great Mills, Maryland. The shooting left three people injured including the shooter. Authorities said the situation was “contained.” (AP Photo/Alex Brandon )